Spin-on-glass (SOG) is frequently used for gap fill and planarization of inter-level dielectrics (ILD) in multi-level metalization structures. It is a desirable material for low-cost fabrication of IC circuits. Commonly used SOG materials may be of two basic types, i.e., an inorganic type of silicate based SOG and an organic type of siloxane based SOG. One of the typical organic type SOG materials is a silicon oxide based polysiloxane which is featured with radical groups replacing or attaching to oxygen atoms. Based on the two basic structures, the molecular weight, the viscosity and other desirable film properties of SOG can be modified and adjusted to suit the requirement of a specific IC fabrication process.
SOG film is typically applied to a pre-deposited oxide surface as a liquid to fill gaps and steps on the substrate. Similar to the application method for photoresist films, a SOG material can be dispensed onto a wafer and spun at a rotational speed which determines the thickness of the layer. After the film is evenly applied to the surface of the substrate, it is cured at a temperature of approximately 400.degree. C. and then etched back to achieve a smooth surface in preparation for a capping oxide layer onto which a second inter-level metal may be patterned. The purpose of the etch-back step is to leave SOG between metal lines but not on top of the metal, while the capping oxide layer is used to seal and protect SOG during further fabrication processes. The siloxane based SOG material is capable of filling 0.15 micron gaps and therefore it can be used in 0.25 micron technology.
When fully cured, silicate SOG has similar properties like those of silicon dioxide. Silicate SOG does not absorb water in significant quantity and is thermally stable. However, one disadvantage of silicate SOG is the large volume shrinkage during curing. As a result, the silicate SOG retains high stress and cracks easily during curing and further handling. The cracking of the SOG layer can cause a serious contamination problem for the fabrication process. The problem can sometimes be avoided by the application of only a thin layer, i.e., 1000.about.2000.ANG. of the silicate SOG material.
In the process of applying a liquid SOG material to a wafer surface, the liquid SOG material is usually fed from a liquid supply bottle in a SOG bottle station. Since the SOG liquid is consumed during the coating process, the level of the liquid in the supply bottle must be continuously monitored. A liquid level sensor, such as a capacitance sensor, is frequently used for such purpose. A conventional SOG bottle station is shown in FIG. 1.
The conventional SOG bottle station 10 shown in FIG. 1 consists mainly of a frame member 12 and a liquid level sensor 24 mounted on one of the horizontal frames 14 of the bottle station 10. A dip tube 8 is immersed in the liquid contained in bottle 16 for the delivery of SOG liquid 22 from the bottle. An open end of the dip tube is normally positioned close to the bottom 18 of the bottle such that most of the liquid stored in the bottle can be utilized.
In the arrangement of the conventional SOG bottle station 10 shown in FIG. 1, the level sensor 24 is normally mounted at a position as low as possible such that most of the SOG liquid 22 stored in the bottle can be syphoned out by the dip tube 8. The maximum usage of the SOG liquid 22 contained in bottle 16 allows a lower fabrication cost. The level sensor 24 which is mounted on the horizontal frame member 14 senses a low liquid level and sends out a warning signal to the machine operator to replace the bottle with a full bottle. When the liquid level sensor 24 is positioned too low relative to the bottle position, a danger of sucking air into the dip tube 8 exists which frequently results in air bubbles being mixed with the SOG liquid 22 and delivered to a wafer surface. When the SOG liquid that contains air bubbles is spin coated on a wafer surface, severe quality problems in the SOG coating are produced which in worst cases, may result in the complete scrap of the wafer. Even when only a minute amount of air bubbles are included in the SOG coating, a part of the wafer must be scraped resulting in a serious yield loss.
In the conventional SOG bottle station 10, the relative positions of the bottle 16 and the fixture frame 12 are fixed such that, in order to change the low liquid level sensed by the liquid level sensor, the position of the sensor itself must be changed which is very difficult. It is therefore desirable to provide a SOG bottle station that allows the position of the bottle be changed with relative ease without disturbing the fixture frame or the position of the level sensor mounted on the fixture frame.
The low liquid level must be sensed by a level sensor at a sufficient low position in the bottle in order to save costs. On the other hand, it must be insured that air bubbles are not syphoned into the dip tube or into the SOG liquid stored in the bottle.
It is therefore an object of the present invention to provide a SOG bottle station for feeding a liquid SOG to a coating apparatus without the drawbacks or shortcomings of the conventional bottle stations.
It is another object of the present invention to provide a SOG bottle station for feeding a liquid SOG to a coating apparatus wherein the elevation of the bottle can be readily adjusted.
It is a further object of the present invention to provide a SOG bottle station for supplying a liquid SOG to a coating apparatus wherein a low liquid level in the bottle can be reliably sensed by a level sensor mounted on a fixture frame of the bottle station.
It is another further object of the present invention to provide a SOG bottle station for feeding a liquid SOG to a coating apparatus wherein the position for sensing a low liquid level in the bottle can be suitably adjusted without adjusting the position of the liquid level sensor.
It is still another object of the present invention to provide a SOG bottle station for feeding a liquid SOG material to a coating apparatus wherein a low liquid level in the bottle can be reliably detected by changing the elevation of the bottle within a range between about 5 mm and about 50 mm.
It is yet another object of the present invention to provide a SOG bottle station for feeding a liquid SOG to a coating apparatus by utilizing a height adjustment means for supporting the liquid supply bottle.
It is still another further object of the present invention to provide a method for adjusting a liquid level in a liquid supply bottle sensed by a level sensor by providing a height adjustment means for supporting a liquid supply bottle thereon such that the elevation of the bottle can be easily adjusted.
It is yet another further object of the present invention to provide a method for adjusting a liquid level in a liquid supply bottle sensed by a level sensor by utilizing a height adjustment means consisting of two sliding blocks of right-angled triangular cross-section slidingly engaging each other on their hypotenuses which supports a liquid supply bottle on top.